Season of birth of schizophrenics in Mississippi, USA.

Prior reviews indicate that schizophrenics tend to be born in the winter, relative to non-psychiatric controls. This conclusion has been criticized, however, as the association between birth seasonality and schizophrenia may be the result of a statistical artifact, the age-incidence effect. To examine this possibility, we studied the birth seasonality of 2892 schizophrenics, controlling for the age-incidence effect. Both before and after instituting these controls, we found excesses for the months of December and March. We conclude that the age-incidence hypothesis does not provide any general explanation of the season-of-birth effect in schizophrenia.